Subject: Re: [PATCH] kexec: Discard loaded image on memory hotplug

From: Baoquan He <[email-masked]>
Date: Sun, 10 May 2020 21:06:46 +0800

Hi James,

On 05/01/20 at 05:57pm, James Morse wrote:
> On x86, the kexec payload contains a copy of the current memory map.
> If memory is added or removed, this copy of the memory map becomes
> stale. Getting this wrong may prevent the next kernel from booting.
> The first kernel may die if it tries to re-assemble the next kernel
> in memory that has been removed.
> 
> Discard the loaded kexec image when the memory map changes, user-space
> should reload it.

As we have discarded in your patches thread, adding a kexec service to
reload kexec should fix this. Do you mean there's still another issue
that we need fix? I may not get it clearly.

> 
> Kdump is unaffected, as it is placed within the crashkernel reserved
> memory area and only uses this memory. The stale memory map may affect
> generation of the vmcore, but the kdump kernel should be in a position
> to validate it.
> 
> Signed-off-by: James Morse <[email-masked]>
> ---
> This patch obsoletes:
>  * kexec/memory_hotplug: Prevent removal and accidental use
> https://lore.kernel.org/linux-arm-kernel/[email-masked]/
> 
>  kernel/kexec_core.c | 40 ++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 40 insertions(+)
> 
> diff --git a/kernel/kexec_core.c b/kernel/kexec_core.c
> index c19c0dad1ebe..e1901e5bd4b5 100644
> --- a/kernel/kexec_core.c
> +++ b/kernel/kexec_core.c
> @@ -12,6 +12,7 @@
>  #include <linux/slab.h>
>  #include <linux/fs.h>
>  #include <linux/kexec.h>
> +#include <linux/memory.h>
>  #include <linux/mutex.h>
>  #include <linux/list.h>
>  #include <linux/highmem.h>
> @@ -22,10 +23,12 @@
>  #include <linux/elf.h>
>  #include <linux/elfcore.h>
>  #include <linux/utsname.h>
> +#include <linux/notifier.h>
>  #include <linux/numa.h>
>  #include <linux/suspend.h>
>  #include <linux/device.h>
>  #include <linux/freezer.h>
> +#include <linux/pfn.h>
>  #include <linux/pm.h>
>  #include <linux/cpu.h>
>  #include <linux/uaccess.h>
> @@ -1219,3 +1222,40 @@ void __weak arch_kexec_protect_crashkres(void)
>  
>  void __weak arch_kexec_unprotect_crashkres(void)
>  {}
> +
> +/*
> + * If the memory layout changes, any loaded kexec image should be evicted
> + * as it may contain a copy of the (now stale) memory map. This also means
> + * we don't need to check the memory is still present when re-assembling the
> + * new kernel at machine_kexec() time.
> + */
> +static int mem_change_cb(struct notifier_block *nb, unsigned long action,
> +			 void *data)
> +{
> +	/*
> +	 * Actions are either a change, or a change being cancelled.
> +	 * A second discard for 'cancel's is harmless.
> +	 */
> +
> +	mutex_lock(&kexec_mutex);
> +	if (kexec_image) {
> +		kimage_free(xchg(&kexec_image, NULL));
> +		pr_warn("loaded image discarded due to memory hotplug");
> +	}
> +	mutex_unlock(&kexec_mutex);
> +
> +	return NOTIFY_DONE;
> +}
> +
> +static struct notifier_block mem_change_nb = {
> +	.notifier_call = mem_change_cb,
> +};
> +
> +static int __init register_mem_change_cb(void)
> +{
> +	if (IS_ENABLED(CONFIG_MEMORY_HOTPLUG))
> +		return register_memory_notifier(&mem_change_nb);
> +
> +	return 0;
> +}
> +device_initcall(register_mem_change_cb);
> -- 
> 2.26.1
> 
> 

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